As we begin a new weekogether after that marathon of strength and fear and courage in boston. And we do have news tonight about the suspect in that bombing. But we're going to begin with late word of a brand new terror plot that was foiled by police. The target was a passenger train, set to travel from new york to toronto, and the arrested men allegedly were getting help from al qaeda terrorists living in iran. We wondered, does this foiled plot have any connection to boston? Pirs thomas starts us off with the latest on that. Authorities made clear the suspected terrorists had deadly intentions and that such an attack could have been catastrop catastrophic. The deadly plot, to derail a busy passenger train in route from new york to toronto, murdering hundreds. Had this plot been carried out, it would have resulted in innocent people being killed or seriously injured. Reporter: The target an amtrak line that partners with canada's via rail. More than four million people use that route every the service crosses one of the most spectacular bridges in north america. Sources tell abc news the plan was to plant bombs and derail the train. By derailing the train, we were looking at a real threat to kill people, hurt people. Reporter: The two arrested suspects had begun surveillance of trains in the toronto area. But authorities had been watching them since august of last year. The ploft was disrupted before it became operational. Here's one of the suspects in handcuffs, escorted today by police. And an unexpected twist. Canadian authorities claim there's a link to al qaeda and iran. The significance of this investigation was that it involved domestic attack planning being supported by al qaeda. Reporter: It's part of al qaeda's playbook. Documents obtained from osama bin laden's secret compound revealed he'd been urging followers to derail a train in the u.S. To kill passengers. And pierre, any connection to boston at all, that we know of? I checked with sources tonight, they say no connection so far. No connection. So it appears to have been on an entirely separate track. Entirely separate. And it shows just how divorce the terrorism threat really is.

This transcript has been automatically generated and may not be 100% accurate.

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